Gasification refers to the conversion of a solid or liquid material, such as a carbonaceous material, into a gaseous fuel. Gasification is of interest for many low-emission technologies in chemical and energy industries.
Gasification of a carbonaceous material can be conceptually divided into two steps although a clear distinction between the two steps is not possible. As the (solid) carbonaceous material is heated up, a mixture of gas and vapour (“volatiles”), including moisture of the carbonaceous material, is released from the carbonaceous material, leaving a solid residue (“char”). Both the volatiles and the char then react with gasifying agents such as H2O and O2 to form a product gas.
Low-rank carbonaceous fuels such as brown coal (lignite), peat, biomass and solid wastes are particularly suitable for gasification due to their high gasification reactivities. However, these low-rank fuels have several specific properties, which must be considered in the design and operation of a gasifier for gasifying these low-rank fuels.
Firstly, low-rank fuels generally have high volatile yields, for example, 80 wt % or more (on dry basis) for some types of biomass. The complete reforming of the tarry components of the volatiles is one of the most important considerations in the design of a gasifier because the removal of tar is cumbersome and costly.
Secondly, low rank fuels often contain well-dispersed alkali and alkaline earth metallic (AAEM) species that can volatilise easily during pyrolysis and gasification. The volatilised AAEM species in the gasification product gas can cause corrosion/erosion of turbine/engine components. The volatilised AAEM species may also react with the bed materials (e.g. sand) in a fluidised-bed gasifier, resulting in the agglomeration and de-fluidisation of the bed materials. On the other hand, if these AAEM species are retained in the char, they can be excellent catalysts for the gasification of char.
Thirdly, char and volatiles from low-rank fuels are very reactive. The interaction between the char and volatiles can enhance the volatilisation of their inherent metallic species (e.g. Na in brown coal and K in biomass), deactivate the char structure and thus reduce the char reactivity. In the worst case, the volatile-char interactions may practically terminate the gasification of char. In the presence of volatile-char interactions, increasing the gasification temperature does not always lead to significant improvement in the gasification rates. In fact, the volatile-char interactions impact almost every aspect of gasification.
The consumption of oxygen is an important consideration in the design and operation of a gasifier to achieve high efficiency. In many gasifiers volatiles, being more reactive than char, tend to react preferentially with O2, leaving the less reactive char to be gasified slowly with steam and other gasifying agents. A more desirable situation would be for the less reactive char to react with O2 enabling the more reactive volatiles to be reformed with steam and other gasifying agents.
The raw product gas may contain traces of tar, volatilised inorganic species (e.g. alkali) and pollutant-forming species (e.g. NH3, HCN and H2S). It normally needs to be cleaned before it can be used, for example, as a gaseous fuel in a turbine/engine or as a feedstock for chemical synthesis. The removal of various undesirable components such as tarry materials, AAEM vapour, particulates and H2S/NH3/HCl from the gasification product gas normally adds to the overall gasification process complexity and forms an important portion of the overall gasification capital and operating costs. When these undesirable species are removed through liquid (e.g. water) scrubbing, a liquid waste stream is generated that must be further treated at great expense. Various conventional catalysts may be employed to reform tar. However, these catalysts often deactivate easily.
There is therefore a need for technological advancement.